fix(core): honor compaction.auto on provider overflow

[clzoc]

Fixes #16882

When provider-triggered context overflow reaches the reactive processor path, compaction.auto: false was being ignored and the session still tried to compact. This change makes the processor stop cleanly with a persisted overflow error when auto-compaction is disabled, while preserving the existing compaction path when it is enabled.

Verification:

• cd /Users/tsunami/Downloads/opencode/packages/opencode && bun test test/session/compaction.test.ts
• cd /Users/tsunami/Downloads/opencode/packages/opencode && bun test test/session/message-v2.test.ts -t "detects context overflow from APICallError provider messages"
• cd /Users/tsunami/Downloads/opencode/packages/opencode && bun test test/session/retry.test.ts -t "does not retry context overflow errors"
• cd /Users/tsunami/Downloads/opencode/packages/opencode && bun typecheck

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found one potentially related PR:

PR #17766 - fix(provider): treat Anthropic billing_error as context overflow for auto-compaction
#17766

This PR is related because it also deals with context overflow and auto-compaction behavior in providers, though it focuses on a different error type (Anthropic billing_error). Since PR #17779 (the current PR) addresses how compaction.auto: false is honored when provider-triggered overflow occurs, these PRs may be addressing related aspects of the same overflow/compaction system but appear to be distinct fixes.

No duplicate PRs addressing the exact same issue were found.

[github-actions]

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